1. Field of the Invention
The present invention relates to a device for magnetically measuring displacement of a moving object and, particularly, a device that includes a stationary magnet member, a movable magnetic member linked with the moving object, a permanent magnet and a magnetic sensor.
2. Description of the Related Art
JP-A-9-236644 discloses a conventional displacement measuring device, which is shown in FIG. 10 of this application. Such a displacement measuring device includes a stationary magnetic member 101, a movable magnetic member 102, a Hall IC (magnetic sensor) 103, a pair of permanent magnets 111, 112 disposed between the stationary magnetic member 101 and the movable magnetic member 102.
The stationary magnetic member 101 has a central projection (or salient pole) 113c projecting toward the movable magnetic member 102, a pair of side projections (or salient poles) 113s projecting toward the movable magnetic member 102 at opposite sides thereof and yoke members 101y formed between the central projection 113c and each side projection 113s. The Hall IC 103 is fixed to the central projection 113c so as to sense magnetic flux density of the magnetic flux passing through the central projection 113c. The permanent magnets 111, 112 are respectively fixed to the side projections 113s and polarized in opposite directions to provide magnetic flux respectively flowing from the permanent magnets 111, 112 through a pair of magnetic circuits J1, J2. For example, the magnetic circuit J2 includes the right side projections 113s, the right yoke portions 101y, the central projection 113c, the Hall IC 103, a first air gap G1, the movable magnetic member 102 and a second air gap G2. The magnetic circuit J1 also has the same members, portion and gaps.
Therefore, the second air gap G2 changes as the movable magnetic member 102 moves in a direction parallel to longitudinal direction of the stationary magnetic member 101, so that the magnetic flux that flows through the Hall IC 103 also changes.
As a result, the Hall IC 103 provides an output signal that corresponds to the travel distance of the movable magnetic member 102. The magnitude of the output signal of the Hall IC is in inverse proportion to the square of the movable air gap G2 and has no linearity, which may sometimes cause difficulty to control a device of a control system.
Further, if the movable member accidentally moves in a direction perpendicular to the longitudinal direction thereof or normal direction, the fixed gap G1 changes, and the magnetic flux to be sensed by the Hall IC 103 increases or decreases from the normal amount of the magnetic flux. As a result, the output signal of the Hall IC 103 may not indicate an accurate position of displacement.